It's possible to remove search results using the "Remove Search Result" feature in the Search SSA, however I can't see any technique to update the setting to remove a previously removed result (eg undo the 'remove' action).
Reference: https://technet.microsoft.com/en-au/library/jj219587.aspx#proc2
In short - we are looking for a way to undo / reverse a "removal" of a search result. It appears as though you can permanently remove a search result but can't re-enable it to be viewable again. Any thoughts welcomed.